The colloidal osmotic pressure of avian plasma proteins is about one-half that of mammals. In light of Starling's hypothesis on the equilibrium between colloidal osmotic and hydrostatic pressures within blood capillaries, it is surprising to find these low values in animals whose arterial blood pressure is higher that that of mammals. The objective of this proposed research is to elucidate the transcapillary fluid exchange in adult birds and its development in embryonic birds by determining the colloidal osmotic pressure of plasma proteins, capillary blood pressures, interstitial pressures and capillary permeability. It should resolve the question of a possible imbalance of pressures at the capillary membranes by comparing colloidal osmotic pressures with values for the capillary blood pressure. The latter is determined by either a micro-puncture or an indirect, micro-occlusion technique. Capillary permeability, also measured with a micro-occlusion method, can be compared with values for mammals. The use of chicken embryos may result in an ideal system to test the activities of various sympatho- and parasympathomimetic agents and drugs which affect capillary permeability.